(May
2006-Oct2008)
What are those strange lights in the sky?
Photos and text by Patrick
Cobb
Figure 1: A lunar display of a 22° halo,
moon dogs, upper tangent arc and a parhelic circle in a cirrus cloud.
One of the
nice tradeoffs about having to endure such long cold winters in interior
Alaska is being able to regularly observe optical phenomena such as halos,
arcs, pillars and sundogs. These phenomena sometimes form in cirrus
clouds high in the atmosphere, but they can also be generated by gently
falling ice crystals, also known as “diamond-dust”, that nucleate
out on clear cold humid days near the surface. Optical phenomena
occur when the ice crystals all possess a common shape and orientation,
causing the scattered light to become concentrated into particular regions
of the sky. The viewing geometry between the observer and the sun
(or moon) also controls whether or not a particular phenomena can be seen.
Figure 2: Light pillars are a common sight in Fairbanks
during winter. These
pillars are originating from the airport runway lights about 1.5 miles
from where this photo was taken. Gently rocking plates behave like
miniature mirrors that reflect the light towards the observer.
The
usual suspects that produce most of the optical phenomena tend to be hexagonally
shaped plates and columns. Plates are responsible for the sun dogs,
parhelic circles and circumzenith arcs, and the columns are responsible
for the 22° and 46° halos as well as the upper and lower tangent
arcs. It
has been shown that both shapes can generate light pillars.
Figure 3 : Extremely cold surface temperatures
generated by strong radiational cooling can cause ice crystals (diamond-dust)
to nucleate out of the air. These crystals tend to be either in the
shape of a plate or prism and can create some spectacular halos. Over
time, these tiny crystals may sediment out and actually accumulate on the
surface. Weather
observations during these events may report “clear with light snow”.
Optical
displays can be formulated theoretically using the basic principles of
reflection and refraction of light waves through a medium, but in addition
to reflection and refraction, diffraction can also cause some rather unusual
sights. Diffraction is the process by which light waves passing around
an object, such as a cloud particle, constructively and destructively interfere
with one another, forming concentric rings around the central light source. Sometimes
the rings contain colors. Such displays are known as corona. The
blur around a street light on a foggy night or the bright light of a full
moon shining through a thin cloud are common corona forming culprits. When
a cloud contains particles of a similar size, the colors of the corona
can be warped and smeared like an oil-slick, causing a phenomenon known
as cloud iridescence.
Figure 4: A circumzenith arc looks like a “smiling” rainbow
situated almost directly overhead, hence the word “zenith”. The
solar elevation angle is less than 32°. On rare occasions, the arc
may make a full ellipse “circum” around the sky.
Figure 5: An example of cloud iridescence. It is most often
observed in alto-cumulus or lenticular clouds but the most spectacular
displays occur in rare nacreous (mother-of-pearl) clouds which form in
the stratosphere in the high latitudes.
(Sept
2005-May2006)
Noctilucent Clouds Seen Around
Fairbanks in August 2005
Photos and text courtesy of Patrick Cobb
Looking north on a late summer evening from
Fairbanks, it is sometimes possible to observe noctilucent clouds
hovering above the horizon. While typical clouds form in the
troposphere below 12km, noctilucents develop in the upper mesosphere
at an astonishing 83km. Like cirrus clouds, noctilucents are
composed of tiny ice crystals, but the processes that initiate
ice deposition at such high altitudes is still unclear. Some
theorize that their formation is tied to interstellar dust fluxes
in the upper atmosphere, while others believe that gravity waves
and atmospheric circulation patterns at lower altitudes provide
the most reasonable explanation for their existence. Water vapor
mixing ratios at this altitude are on the order of 3ppm (4K to
10K times drier than the lower troposphere) so extremely cold
temperatures (~150K) are needed in order to achieve supersaturation
in this dry environment.
People first took notice of noctilucents over
Europe during the summer of 1884. Coincidentally, this was only
a few months after the famous Krakatoa eruption in Indonesia.
The massive eruption spewed millions of tons of dust and ash
into the upper atmosphere, resulting in a 1.2C drop in average
global temperatures and triggering an artist's feast of colorful
sunsets for many years after the eruption. Many believed that
the Krakatoa eruption was directly responsible for the subsequent
noctilucent sightings, but this theory has since been proven
false.
Because they are so optically thin, noctilucents
will only appear under particular illumination conditions. Only
when the sun drops to between 6-12 degrees below the horizon
can the clouds become visible from the ground. At these angles,
the sky is almost completely dark to the viewer, but at 85km,
the clouds sit in the path of direct sunlight, making them look
ethereal, like bright apparitions of fine cobweb fabric: hence
the name noctilucent‚ or night shining.‚
Sightings of noctilucents have been increasing
in recent years, leading some to speculate that anthroprogenic
pollution, mainly the increase in atmospheric methane, are contributing
to their formation. The photodissociation of methane, a powerful
greenhouse gas, by UV light provides an important source of water
vapor in the upper atmosphere. Since methane concentrations have
been steadily rising as a result of human activities, some connect
global change with the recent increase in noctilucent sightings.
Unfortunately the mesosphere is beyond the range
of weather balloons, yet it is too low and too dense for satellites
to pass. As a result, sampling this part of the atmosphere directly
poses a considerable challenge. However, ground based systems
like LIDAR instruments offer researchers the opportunity to study
noctilucent clouds remotely. By measuring the intensity and the
polarization of backscattered light, scientists are able to infer
many of their physical properties like cloud thickness, optical
depth, and particle size and shape. |
